(1) Field of the Invention
The present invention refers to a document size detection device, for use in an image forming apparatus or the like, which detects a size of a document placed on a platen. More precisely, the document size detection device comprises a multiplicity of reflection-type light sensors inside a document cover, wherein a light emitted from a light emitting element of each sensor is projected on the document and the reflected light is detected by a receiving element of the sensor, whereby the document size is detected.
(2) Description of the Related Art
A document size detection device is applied to, for example, a copier. Usually, the copier equipped with a document size detection device has a multiplicity of feeding cassettes for accommodating papers of different sizes (A4, B4, etc.). The appropriate-sized paper for the document is automatically selected and fed, whereby smooth and accurate copying is effected.
FIGS. 1a and 1b show a document size detection portion of such a copier. The copier comprises a document cover 1000 for pressing a document d on a platen 1100. The document cover 1000 has a pressing sheet 1200 formed of a white resin film on a bottom surface thereof, and the sheet 1200 has multiple detection holes 1200a at specified positions. (FIGS. 1a and 1b show only one detection hole 1200a.) Each detection hole 1200a is covered with an infrared ray transmitting filter 1120, which is formed of a material that transmits infrared rays but shields visible lights.
The document cover 1000 contains multiple sensors F above the holes 1200, respectively, inside the document cover 1000. (FIGS. 1a and 1b show one of the sensors.) Each sensor is equipped with an infrared ray emitting element 1110a and an infrared ray receiving element 1110b. When an infrared ray emitted from the emitting element 1110a is reflected on the document d and received by the receiving element 1110b as shown with optical paths R.sub.0 and R.sub.1, the sensor F detects the document d in a detecting area thereof. The document size is judged by which sensor(s) detects the document d in the detecting area(s) thereof.
The copier has a scanner 200 below the platen 1100, the scanner being movable in a direction of an arrow e. The scanner 200 comprises an exposure lamp 210 and a mirror 220. A visible light emitted from the exposure lamp 210 is reflected on the document d and then on the mirror 220 as shown with optical paths R.sub.2 and R.sub.3 and guided to an image forming section (not shown) to expose a photoconductive drum.
Usually, it is desirable that the infrared ray transmitting filter 1120 has a smooth surface in order to enhance the infrared ray transmitting efficiency. If there is no document at the corresponding portion with the hole 1200a (FIG. 1b), a visible light emitted from the exposure lamp 210 (.sub.R 4) is mostly regularly reflected on the filter 1120 (R.sub.6). That means less quantity of light follows an optical path R.sub.5, which is the ideal path to the photoconductive drum. As a result, the filter 1120 covering the hole 1200a is misread as an image and a shade is formed in a portion of the paper which corresponds with the position of the hole 1200a, namely, on the margin. This occurs when a paper having an image copied thereon has a margin, for example, when the document d is copied on a paper which is larger than the document d or when the document d is reduction-copied on a same-sized paper.